FIG. 1 shows a conventional magnetic-particle-type coupling device disclosed in the Japanese Patent Application (OPI) No. 60733/81 (the term "OPI" as used herein means an "unexamined published application"). Rotation whose speed is changed by a transmission 2 to which the coupling device 1 is connected is decelerated by the differential gear unit of a speed reducer 3 and then transmitted to axles. The magnetic-particle-type coupling device 1 comprises a driving body 4 constructed by conjoining two members to each other in the axial direction of the coupling device and secured to a driving plate 6 secured to a crankshaft 5; an excitation coil 7 housed in the opening of the driving body 4; a driven body 8 whose outside circumferential surface faces the inside circumferential surface of the driving body 4 across a gap and which is secured to a hub 11 supported by a bearing 10 to a bracket 9 attached to the driving body; magnetic particles 12 filled in the gap; a pair of slip rings 13 which are secured with an electric insulation bushing 15 to a cap 14 attached to the bracket 9 and are connected so as to supply electricity to the excitation coil 7; a brush unit 16 for supplying the electricity through the slip rings 13; and a coupling shaft 17 secured to the hub 11 and constituting the input shaft 17a of the transmission 2. A case 18 houses the coupling device 1 and the transmission 2. A temperature sensor 19 is secured to the portion of the case 18, which faces the coupling device 1. Shown at 20 and 21 in FIG. 1 are a ring gear and the output shaft of the transmission 2, respectively.
When the electricity is supplied to the excitation coil 7 as the driving body 4 is rotated by the crankshaft 5 after the starting of an engine, the magnetic particles 12 are magnetized to couple the driving body and the driven body 8 to each other to transmit the rotation of the driving body to the driven body to rotate the input shaft 17a of the transmission 2 through the coupling shaft 17. Between the input shaft 17a and output shaft 21 of the transmission 2, the speed of the rotation is changed among a low, a second, a third and a fourth speed stages by a plurality of speed change gear means between the input and the output shafts in response to the manipulation of a manual speed change lever. The rotation of the output shaft 21 is transmitted to wheels through the differential gear unit of the speed reducer 3 and the axles.
Since it is necessary to smoothly increase the coupling torque of the magnetic-particle-type coupling device 1 at the time of starting of a vehicle, a slip takes place between coupling members to generate heat to raise the temperature of the coupling device. If the rise in the temperature of the coupling device is large, the device burns. This is a problem.
To prevent the problem, the temperature sensor 19 is attached to the case 18 to supply a signal from the temperature sensor to a controller through a second speed stage switch which is turned on when the vehicle is started at the second speed stage of the transmission 2. When the second speed stage switch is turned on and the temperature in the case 18 has exceeded a limit due to the frequent repetition of the starting of the vehicle at the second speed stage of the transmission 2, the signal from the temperature sensor 19 is supplied to the controller to alter the electrical current of a clutch to shift the torque of the clutch to a level for a lower speed stage. As a result, the starting of the vehicle at the second speed stage of the transmission 2 is made impossible to suppress the rise in the temperature in the case 18 to prevent the coupling device 1 from burning.
However, since the temperature of the internal opening of the case 18 is detected by the temperature sensor 19, it is difficult to accurately detect the temperature of the driving body 4 or the driven body 8. Particularly when the temperature of the driving body 4 or the driven body 8 has exceeded a limit in a short time, the detection of the rise in the temperature by the temperature sensor 8 delays so that the coupling device 1 burns. This is another problem. Although it is possible to attach cooling fins or the like to the driving body 4 in an outer position to cool the driving body, it is difficult to cool the driven body 8 in an inner position. This is still another problem.
A conventional cooler for a magnetic-particle-type coupling device was disclosed in the Japanese Utility Model Application No. 510/85. In the cooler circumferential surface of a stator in an internal position is secured to the hermetically sealed tube of a heat exchanger, cooling fins are provided on the outer end portion of the tube and project into the external air, a wick is provided in the tube, and a working liquid is hermetically enclosed in the tube. Although the stator in the internal position can be cooled by the external air through the heat exchanger of the cooler, the efficiency of heat transfer by a means for transferring the heat of the stator to the external air through the cooling fins of the heat exchanger is low and the cooling fins need a large heat transfer area which means that the installation space for the cooler is large. These are still other problems.
The present invention was made in order to solve the above-mentioned problems.
Accordingly, it is an object of the present invention to provide a magnetic-particle-type compact coupling device in which a driven body is directly cooled through efficient transfer of heat to enable the coupling device to withstand severe operation.